Where's the evidence of this statement? It's well known that blood glucose monitors get a wide latitude for accuracy from the FDA anyways so real accuracy would be very difficult to determine.
Why would real accuracy be difficult to determine? Clinicians get to test these on their patient populations and validate against other high accuracy methods (point-in-time serum draws). For instance, Dr. Peter Attia says he gets a useful fraction of his clinic to use CGMs (as well as personal use), and over years sees a strong signal.
There are many other Drs that have gone on record.
Anecdotally - I helped a T1 diabetic friend set up Loop using a Dexcom G5 and then a G6. Over the course of many months of use, the G6 has proved MUCH more reliable and accurate than the G5, which resulted in far better Looping. The Loop community as a group seem to confirm this increased accuracy.